1. Field:
This invention relates to the field of sterilization systems. More specifically, this invention comprises a combination unsaturated chemical vapor sterilization system and a steam sterilization system utilizing the same sterilizing chamber. It specifically provides a combination temperature sensor/pressure relief valve for such systems.
2. State of the Art:
As discussed in the pamphlet entitled "Principles and Practice of Unsaturated Chemical Vapor Sterilization", published by Aseptic-Thermo Indicator Medical-Surgical Division of the Parke, David & Co., of North Hollywood, Calif., there are four generally accepted sterilization systems: steam, dry heat, chemical agents and unsaturated chemical vapor. Traditionally, these sterilizing methods utilize separate devices to sterilize materials.
The steam sterilization autoclave is the most commonly used form of sterilization. Generally, the steam autoclave exposes materials to be sterilized to saturated steam at 121 degrees centigrade and 15 pounds per inch pressure for 12 to 15 minutes. The temperature and pressure may be varied to alter the exposure time necessary for sterilization. Pressurized steam in a vacuum has the advantage of quick penetration of items to be sterilized such as textiles. The moisture also has the advantage of more quickly killing spores. The disadvantage of steam is that it rusts and corrodes certain metals, dulling cutting edges of many instruments and requiring the drying of objects after the completion of the cycle. Also, certain plastics, rubbers and other materials sensitive to heat and moisture cannot practically be sterilized in a steam autoclave. Thus, one of the other types of sterilizers is required for these materials.
A dry heat sterilizer avoids the rusting and corroding problems caused by saturated steam. However, high temperatures, frequently at 160 to 170 degrees centigrade, for relatively long exposure times of two to four hours are required. Also, the dry heat tends to "stratify" or trap air in layers. The layers create air pockets which prevent the even exposure to heat.
One commonly used chemical sterilizer uses ethylene oxide. Ethylene oxide gas sterilizers are effective sterilizing equipment if sophisticated handling techniques are used. Pure ethylene oxide gas is flammable and toxic. Most gas sterilizers use a non-flammable mixture of twelve percent (12%) ethylene oxide gas and eighty-eight percent (88%) chloro and/or fluro methane gases. The main disadvantage of ethylene oxide sterilizers is the long sterilization times required--typically one to four hours exposure time plus additional time for aeration. Certain porous materials have aeration times as long as five to seven days. Ethylene oxide is also a known mutagen and carcinogen requiring extreme care in handling.
Another commonly used chemical sterilizer uses liquid alkaline gluteraldehyde. This disinfectant solution is desirable where higher temperatures will damage the materials to be sterilized. The disadvantage of gluteraldehyde sterilants is that they are unstable in solution. They must therefore be mixed prior to use. Also, the effective life of the solution is approximately two weeks. Exposure times of a minimum of ten hours are required, and items must be thoroughly rinsed to prevent damage to skin or other sensitive tissues.
The unsaturated chemical vapor system is used primarily to sterilize metal instruments and materials. The unsaturated chemical vapor system uses heat, water and chemical synergisms. Moist heat is used to induce chemical reactions which denature and coagulate proteins by catalysis in the presence of water. To prevent the rusting, corroding and dulling effect of water, various solutions of alcohols, acetones, ketones and formaldehyde are used with water to reduce the water content below approximately fifteen percent (15%) by weight. Ethyl alcohols and formaldehydes in combination especially act as effective disinfectants. An example of these chemical solutions is the solution of isopropyl alcohol, methyl alcohol, ethyl alcohol, formaldehyde, acetone, methyl ethyl ketone, butanol, water and fragrances produced and sold by MDT Chemical Company of Gardena, Calif., under the trademark "Vapo-Steril". The "Vapo-Steril" percentages for isopropyl alcohol, methyl alcohol, ethyl alcohol and butanol are approximately 82%; formaldehyde approximately 1%, and water, ketone and acetone approximately 17%.
The unsaturated chemical vapor sterilizer mechanically introduces a fixed amount of chemical solution into a preheated, chamber. The chamber is pre-evacuated of air prior to the introduction of the chemical solution. The air is evacuated because it can interfere with sterilization. Also, the emptying of the air from the chamber enables higher concentrations of sterilizing vapors to be attained within the chamber. Evacuation is generally accomplished by applying a vacuum of minus 20 to 30 mm of mercury. This solution generally vaporizes at approximately 78 degrees centigrade. When heated to approximately 131 degrees centigrade, plus or minus five degrees centigrade, a minimum sterilizing pressure of approximately 20 pounds per square inch is attained. This temperature and pressure is maintained for approximately 20 minutes.
Sterilization is begun immediately upon admission of the chemical solution into the preheated chamber. The chemical solution condenses on the materials to be sterilized which have not been preheated. Preferably, this is accomplished by loading the sterilizer carrier tray with items to be sterilized which have been rinsed in cold water and towel dried. The tray is then placed in the preheated sterilization chamber and the chemical solution immediately introduced. The solution condenses on the colder materials and starts bactericidal activity and rehydrating desiccated spores. Rehydrated spores are more susceptible to sterilization in shorter periods of time. As the chamber is heated, it is pressurized to approximately 20 pounds per square inch. The condensed solution vaporizes, resulting in increased sterilization activity.
Following sterilization, the chamber is depressurized and air is pumped through a sterile filter into the chambers and serves to purge residual chemicals from the chamber into a tank and finally through a chemical filter.
The unsaturated chemical vapor sterilizer has the advantages of shortened sterilizing times and prevents rusting, corroding and dulling of metal objects. The vapor sterilizer has the disadvantage of slowly penetrating textiles and similar woven materials for sterilization. Also, it cannot be used with objects such as certain rubber and plastic objects which are attacked by the chemical solutions. Nor can it be used with objects which cannot withstand a temperature of 131 degrees centigrade.
Thus, to sterilize a wide variety of materials, different sterilizers were required depending upon the necessary sterilizing conditions. Several attempts to combine pre and post sterilizing operations utilizing the same sterilizing chamber have been attempted with varying degrees of success. Young et al, U.S. Pat. No. 4,193,818, entitled "Combined Ultrasonic Cleaning and Biocidal Treatment in a Single Pressure Vessel", combined an ultrasonic cleaning circuit with a biocidal sterilizer. The ultrasonic cleaning circuit's function was to prepare items for sterilizing by first removing soil, blood, debris and other extraneous materials before the sterilizing cycle is operated. After the ultrasound treatment, the chamber is drained and the articles to be sterilized rinsed. A biocidal fluid, e.g., steam, is then injected into the sterilizing chamber to sterilize the articles placed therein. This invention does not utilize the same sterilizing chamber for different sterilizing operations.
Other devices utilize the same sterilizer chamber for a drying cycle after a steam sterilization cycle. The dry heat cycle is carried on in the sterilizer vessel at room pressures. This type of device rusts, corrodes and dulls certain metal objects. Use of the drying cycle at room pressures also results in extremely long drying times.
Another device utilizes the same sterilizing chamber for steam sterilizing, or alternatively, for ethylene oxide sterilizing. Presumably, the ethylene oxide cycle is used for objects which are sensitive to heat. However, the ethylene oxide cycle takes a long time to operate, and is very toxic and flammable requiring extreme care in handling.
It is important to control the temperature and pressure conditions within the sterilizer's chamber in all of the previously described sterilizers.